To bond electronic components such as semiconductor devices to a substrate such as a glass epoxy substrate, various methods have been proposed to use a low-melting-point metal paste of a variable-melting-point type, whose melting point changes through heat treatment, for suppressing distortion resulting from a difference in coefficients of thermal expansion between the electronic components and the substrate, and achieving a bonding quality (see Japanese Laid-Open Patent Publication No. 2002-261105, Leaflet of WO No. 2007/125861, Japanese Laid-Open Patent Publication No. 2006-102769 and Japanese Laid-Open Patent Publication No. 2008-161881).
For instance, as illustrated in FIG. 1, a low-melting-point metal paste 10 of a variable-melting-point type, which is a conventional electroconductive bonding material, is composed of metal components of a high-melting-point metal particle (Sn—Bi-plated Cu particle) 1 and a low-melting-point metal particle (Sn—Bi particle) 2, and by a flux component 3. The initial melting point of this low-melting-point metal paste 10 of the variable-melting-point type is 139° C. based on the melting point of the low-melting-point metal particle (Sn—Bi particle). When the low-melting-point metal paste 10 of the variable-melting-point type is heated at 150° C. or lower, Sn in the low-melting-point metal particle (Sn—Bi particle) 2 diffuses into the high-melting-point metal particle (Sn—Bi plated Cu particle) 1 to form a Bi-segregated layer 4 and a Cu—Sn-based intermetallic compound 5 and be converted into a metallic bonding state, and thereby the melting point of the low-melting-point metal paste 10 of the variable-melting-point type rises to approximately 250° C. or higher.
Because the melting point of the low-melting-point metal paste of the variable-melting-point type, which is the conventional electroconductive bonding material, thus changes through the heat treatment, the heating temperature which may be selected in an electroconductive bonding-material supply step is restricted to the initial melting point or lower which is the melting point of the low-melting-point metal particle (Sn—Bi particle). As illustrated in FIG. 2, when a substrate 6 and electronic components 8 are bonded to each other, if the low-melting-point metal paste 10 of the variable-melting-point type, which is the conventional electroconductive bonding material, is supplied onto the electrode 7 of the substrate and is heated to deposit onto the electrode of the substrate, the low-melting-point component disappears and the melting point rises to 250° C. or higher. Because of this, the electroconductive bonding-material supply step of previously depositing the low-melting-point metal paste of the variable-melting-point type onto the electrode of the substrate while supplying the material onto the electrode may not be selected, and the problem is that the electronic components may not be mounted on the substrate of the electronic components at a low temperature of 150° C. or lower.
In addition, as illustrated in FIG. 3, when the low-melting-point metal paste of the variable-melting-point type 10, which is the conventional electroconductive bonding material, is charged into the recessed part of a countersunk jig 31, the terminal of the electronic components is pressed to the paste, the resultant product is heated at 150° C. or lower, and the paste is heated/transferred (deposited) onto the terminal 9 of the electronic components 8, the low-melting-point component disappears and the melting point rises to 250° C. or higher. Because of this, the heating and transferring step of once melting the electroconductive bonding material and transferring the material onto the terminal of the electronic components may not be selected, and the problem is that the electronic components may not be mounted on the substrate of the electronic components at a low temperature of 150° C. or lower.
Accordingly, the electroconductive bonding-material supply step of depositing the electroconductive bonding material onto the electrode of the substrate while supplying the material onto the electrode, and the transferring step of once melting the electroconductive bonding material and transferring the material onto the terminal of the electronic components may not be selected, because the low-melting-point component in the conventional electroconductive bonding material disappears by the heat treatment and the melting point rises to 250° C. or higher, and a low-temperature bonding at 150° C. or lower is difficult under present circumstances.    [Document 1] Japanese Laid-Open Patent Publication No. 2002-261105    [Document 2] Leaflet of WO No. 2007/125861    [Document 3] Japanese Laid-Open Patent Publication No. 2006-102769    [Document 4] Japanese Laid-Open Patent Publication No. 2008-161881